The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission at an extremely low vehicle speed range when a high engine speed driving range (Ds range) is selected.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency o driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
When starting the vehicle, the transmission ratio is set at a maximum value. When the engine speed exceeds a predetermined value, an electromagnetic clutch engages, so that the vehicle is started. When the vehicle speed and engine speed exceed set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced, even if the engine speed is kept constant. Thus, the vehicle speed increases with the reduction of the transmission ratio.
In a transmission ratio control system disclosed in Japanese Patent Laid-Open 61-48659, an actuator is provided for applying a certain load on a spool of the transmission ratio control valve, for providing a high engine speed driving range (hereinafter called Ds range). Accordingly, in the Ds range, the transmission ratio changes in a higher engine speed than the drive range (D range). For example, when the Ds range is selected under a driving condition where the transmission ratio changes as shown by a line m.sub.8 of FIG. 6, the transmission is downshifted so as to vary as shown by a transmission ratio changing line m.sub.5. Accordingly, in order to maintain the vehicle speed at that time, the engine speed must be increased. Thus, engine torque is increased, and engine braking effect is also increased. Additionally, when the transmission ratio exceeds a predetermined transmission ratio S, the load exerted on the spool by the actuator is gradually decreased by a correcting means and a changing means so that the transmission is upshifted at low engine speed as shown by a transmission ratio changing line m.sub.7, thereby preventing an extreme engine braking effect.
However, in the system, the construction is complicated by the addition of the changing means.
Moreover, even though the driveability is improved in a low and middle vehicle speed range because of the reduction of the engine braking effect, in a range between a low vehicle speed and an extremely low vehicle speed slightly higher than a clutch disengagement speed, the engine braking effect is excessively large. Accordingly, the vehicle stumbles due to large engine braking or shock caused by the disengagement of the clutch.
On the other hand, the transmission system has the following function during release of an accelerator pedal in an extreme low vehicle speed range. When engine speed decreases because of an increase of load on the engine when releasing of the accelerator pedal, the transmission is downshifted by the operation of the transmission ratio control valve. The load on the engine reduces because of the downshifted ratio, so that the engine speed increases. When engine speed increases, the transmission is upshifted. When the transmission is upshifted, the load increases, thereby reducing the engine speed. The oscillation continues until a balance is made in the system. If such an oscillation of engine speed occurs, the engine speed varies greatly because of the steep inclination of the line. As a result the vehicle jerks.